Currently available designs for launching plugs downhole employ a variety of mechanical retention devices for sequential release of plugs. These devices are typically collets or a variety of shearable devices intended to sequentially release wiper plugs into a liner ahead of the cement and behind it. Typically, these devices are attached at the bottom of the liner setting tool. After the liner is properly hung, the tool is called upon to sequentially launch the wipers to facilitate the cementing of the liner through a cementing shoe. Typical of such products is the LFC four-plug system, product No. 269-27 made by Baker Oil Tools. In this device, a series of darts of different sizes engage different sized wiper plugs to pry them loose from the connection mechanism. The dart goes down with the wiper plug and must be drilled out later. A shock loading is placed on the retention mechanism as the dart lands in its respective plug. While there are many variations of these wiper plug-launching systems, the nature of the retention devices used for the wiper plugs has in the past caused some operational difficulties in a variety of different ways. In some situations, the wiper plugs would not release at all. In other situations, more than one wiper plug released when only one was intended to be released. Darts used to launch the plugs had to be drilled out.
Typically in these systems, after landing the first wiper plug, a barrier through or around the plug is broken to allow pumping of the cement before pushing the next plug down behind the cement. Plugs with breakable barriers for this purpose have been used in the past. These plugs are also typically made of soft materials so that they can be quickly drilled out after the cementing operation is concluded. Typical of such plugs are those illustrated in U.S. Pat. Nos. 5,435,386; 5,361,835; and 5,311,940.
It is an object of this invention to provide a reliable apparatus for launching objects downhole, particularly wiper plugs in the proper sequence with confidence. In describing and claiming the invention, references to "wiper plugs" or "plug" are intended to be broad enough to include any other objects such as "balls," as one example. This objective is accomplished by regulated movement of the various components to avoid abrupt movements due to pressure buildups normally used in delivery of wiper plugs where darts land in them in order to launch. The objective is further met by a sequential operation which can effectively launch one or a plurality of plugs in a desired sequence. Provisions are made for a pressure-assisted shear release as an emergency technique for release of the wiper plug in the event it does not automatically release for any reason. These and other objectives of the invention will become more readily understandable to one of skill in the art from a review of the preferred embodiment described below.